intertidal mud 1  220x136 hhWhat are intertidal mud flats?

Intertidal mud flats can be defined as shallow-sloped shoreline, with expanses of fine sediment. They are often components of estuaries, and are revealed when the tide goes out. Large mud flats may seem to stretch to the horizon, and reflect the colours of the sky in the convoluted patterns of shallow pools of water. Despite their empty appearance, mud flats are valuable as habitat and feeding grounds for many species of wildlife.

Where are intertidal mud flats found in the CRD?

The contemporary shoreline around the region looks much different than it did 200 years ago, when intertidal mud flats were much more common. For example, mud flats probably existed at locations such as: adjacent to a salt marsh located where the Empress Hotel now stands; at the mouth of a creek that once discharged into Rock Bay; and in an inlet (now filled in) adjacent to Point Hope, on the west side of the Johnson Street Bridge. Before seawalls were constructed in the Gorge Waterway and Portage Inlet, the mud flats there were much larger. Expansive mud flats also existed in Lang Cove in Esquimalt Harbour, before these were filled in for industrial uses.

Fortunately, some intertidal mud flats remain in the CRD harbours and are located:
  • in Esquimalt Harbour, near the mouth of Millstream Creek, in Plumper Bay, and Thetis Cove
  • at the mouth of Craigflower and Colquitz creeks in Portage Inlet
  • in West Bay and the Selkirk Water (near the mouth of Cecelia Creek) in Victoria Harbour
  • at the mouth of Colwood Creek in Esquimalt Lagoon, and at the entrance to the lagoon itself.

How do intertidal mud flats form?

Mud flats form when sediment, carried by rivers or the ocean, encounters a low-energy environment and settles to the bottom. Inlets or bays that are sheltered from waves are some examples. Over time, this sediment accumulates, making the area flatter and wider, which in turn encourages further sediment deposition.

What lives in mud flats?

At first glance, mud flats seem quite devoid of life. However, a closer look proves otherwise. The fine sediments have very little space between them, preventing oxygen from diffusing below the top few centimetres. A unique ecosystem of animals (e.g. clams, mud shrimp, worms) thrives in the anoxic conditions common below the surface of mud flats. Due to the constant shifting of sediment in the currents, few large plants grow there. One exception is eelgrass, which grows close to and below the low tide line, down to a water depth of about 6 metres. Eelgrass in turn hosts an array of animals including herring, crabs, snails and juvenile salmon.

Eelgrass and diatoms (single-celled algae with silicon shells) are the main primary producers of mud flats. In other words, they produce organic matter (i.e. plant tissue) from carbon dioxide and water, using photosynthesis. This material provides fundamental nutrients for other species.

Due to the high organic content of the mud, bacteria are very prevalent, even in the low-oxygen environment below the surface. They perform a vital service in decomposing the plant matter and making it more digestible as food for other organisms. Invertebrates such as nematode worms and protozoa feed on bacteria, and larger organisms such as crustaceans (e.g. amphipods, mud shrimp), mollusks (e.g. clams, snails, native oysters) and polychaete worms in turn feed on the microorganisms and on detritus (decaying plant and animal matter).

Moving up the food web, small fish and birds prey on crustaceans and mollusks, and these fish are preyed upon by larger fish, birds of prey, seals and otters. Some of the many types of shorebirds found in mud flats include sandpipers, plovers, Lesser Yellowlegs, Longbilled Curlews, Short-billed Dowitchers, and Common Snipe. Great Blue Herons may also be seen stalking fish in the shallows.

Why are mud flats important?

Migratory birds rely on the bonanza of worms and other invertebrates in mud flats to gain the fat reserves necessary for long-distance flights. Millions of shorebirds stop along the shorelines of BC to feed. The mud flats of Esquimalt Lagoon, for example, provide one of the most important migratory bird stop-over areas in the CRD.

Mud flats with eelgrass meadows provide spawning grounds for herring, which provide food for marine mammals, large fish and sea birds.

Some of the shellfish (e.g. clams and oysters) that live in mud flats have been or are still harvested by people.

The plentiful bacteria in mud flats help to break down contaminants from urban runoff, such as heavy metals, hydrocarbons (oil, gasoline, solvents) and other organic chemicals. In excessive amounts, however, these substances can accumulate in the bodies of invertebrates and subsequently the animals that eat them.

Many animals depend on mud flats either directly, for habitat, or indirectly, for prey. Mud flats are important for the overall integrity of the marine food web. <2>What threatens mud flats?

In the recent past, mud flats were often been mistakenly regarded as “waste lands” that were useless until modified for more productive purposes. They have been filled in for urban development and agriculture, and dredged for shipping channels. These activities continue to threaten some mud flats, although their value is now being recognized and efforts taken to limit the impacts of development.
  • Dredging affects mud flats by destroying habitat and altering the pattern of sediment movement. Disturbed or dumped sediment can smother eelgrass beds and bottom-dwelling species.
  • Construction of marinas, docks and wharves can also disturb sediments, “shade out” eelgrass beds, and create a pathway for pollutants, for example from spilled fuel. Propeller wash also disturbs sediment, and may contribute to erosion of mud flats.
  • Erosion in upland areas can introduce excessive sediment into mud flats that may smother bottom-dwelling plants and animals. Erosion can be caused by construction, logging and road-building activities.
  • When rain runs off from impervious surfaces (e.g. buildings, roads and parking lots), instead of gradually soaking into the soil, it picks up pollutants and rushes into streams with more force. This can erode stream channels, and transport pollutants into the marine environment.
  • Excessive nutrients, from sewage contamination or fertilizers, can cause algae blooms around mud flats. The growth of algae is stimulated by the nutrients, but when the plant material decays, dissolved oxygen in the water is used up by bacteria. This robs the water of oxygen, to the detriment of fish and bottom-dwelling organisms.
  • Trampling of vegetation by people and animals.
  • Invasive species are another result of development in and around mud flats. Invasives often do not have predators or pathogens in the areas where they are introduced. This allows them to out-compete native species. Examples include the Japanese oyster, which can crowd out the smaller native Olympia oyster, and Spartina, a marsh grass that has not yet established in BC but which threatens intertidal mud flats in many regions of the US.

How can I help protect mud flats?

For information on protecting mud flats, please visit our How Can I Help section.

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© Image courtesy of Bill Irvine

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